Field of the Invention
The invention relates to a device for transferring a sheet via a peripheral region or area of a cylinder, which includes an airflow generator for applying an airflow onto the sheet-carrying peripheral region of the cylinder, and to a method of transferring a sheet via a peripheral region of a first cylinder and for feeding the sheet in an exact position to a second cylinder disposed downline from the first cylinder, wherein at least one airflow is directed onto the sheet lying on the peripheral region of the cylinder.
Such devices and methods of transferring a sheet have become known heretofore from, for example, the published German Patent Document DE 195 47 580 A1. The transfer device is installed in a sheet-fed printing machine, which can be operated in recto printing or in recto/verso or perfector printing and serves for transferring sheets from a first sheet-guiding cylinder to a third sheet-guiding cylinder in exact register. This is realized with the aid of a peripheral region of a cylinder which, in this connection, is also designated as a storage drum. It has been shown that, at high printing throughputs, i.e., at a high machine speed, the sheet engaging the periphery of the cylinder begins to have waves formed therein, which impairs in-register sheet guidance. The cause thereof is both centrifugal force acting upon the sheet, as well as a negative pressure zone which forms over the sheet because of the relative speed between the rotating cylinder and the stationary ambient air. In order to avoid this disadvantage, in heretofore known transfer devices, an airflow directed counter to the direction of rotation of the cylinder is blown onto the peripheral region of the cylinder presenting the sheet, and thus spreads out smoothly on the cylinder the sheet that has become deformed with the waves. It has been shown, that even through this measure, smooth contact between the sheet and the cylinder cannot always be assured.
It is therefore an object of the invention to provide a sheet transfer device and a method for transferring a sheet of the type referred to in the introduction hereto wherein smooth contact between the sheet and the peripheral region of the cylinder is assured, in particular, even at high machine speeds.
With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, a device for transferring a sheet via a peripheral region of a cylinder, comprising an airflow-generating device for applying at least one airflow to the peripheral region of the cylinder, with which the sheet is associated, the airflow being directed in a direction of rotation of the cylinder.
In accordance with another feature of the invention, the airflow has a speed which is at most as great as the speed of the sheet.
In accordance with a further feature of the invention, the airflow is inclined with respect to an imaginary tangent to the periphery of the cylinder at an angle 0xc2x0 xe2x89xa6xcex1 less than 90xc2x0 , the tangent intersecting a point of impingement of a central flow line of the airflow.
In accordance with an added feature of the invention, the airflow-generating device serves for producing compressed air or blast air.
In accordance with an additional feature of the invention, the airflow-generating device is assigned to an air guide device for aligning the airflow.
In accordance with yet another feature of the invention, the air guide device has at least one blast box formed with at least one air outlet opening.
In accordance with an alternative feature of the invention, the air guide device has at least one blast tube formed with at least one air outlet opening.
In accordance with another alternative feature of the invention, the air guide device has at least one blast box and at least one blast tube, respectively, formed with at least one air outlet opening.
In accordance with yet a further feature of the invention, the cylinder is formed of a storage drum of a reversing or turning device of a sheet-fed rotary printing machine.
In accordance with yet an added feature of the invention, the storage drum has at least one holding device for the sheets, and preferably at least one holding device for the leading edge or at least one holding device for the trailing edge of the respective sheets.
In accordance with yet an additional feature of the invention, the storage drum has a first holding device for a leading edge of the respective sheets, and a second holding device for a trailing edge of the respective sheets.
In accordance with a concomitant aspect of the invention, there is provided a method of transferring a sheet via a peripheral region of a first cylinder, and of feeding the sheet in an exact position to a second cylinder disposed downline of the first cylinder, wherein at least one airflow is directed onto the sheet lying on a peripheral region of the cylinder, which comprises directing the airflow in the direction of rotation of the cylinder, and preferably directing the airflow onto the sheet from a transfer device.
In order to achieve the objective of the invention, a transfer device is provided which includes a cylinder to which an airflow-generating device has been assigned, by the aid of which at least one airflow can be applied to a peripheral region of the cylinder on which the sheet to be transferred is disposed. The transfer device is distinguished by the fact that the airflow is directed in the direction of rotation of the cylinder. Due to this measure, it is possible to ensure that the air located over the sheet lying on the periphery of the cylinder moves in the same direction as the sheet. Due to the reduced differential speed between the sheet and the air located above it, the pressure in the negative pressure zone formed by the relative speed between sheet and air is so great that smooth contact between the sheet and the periphery of the cylinder can be realized.
In an advantageous exemplary embodiment of the transfer device, the velocity of the airflow is the same as the speed of the sheet resting on the periphery of the cylinder. The differential speed between the airflow and the sheet is therefore zero, i.e., the airflow over the sheet is at rest relative to the sheet, so that no negative pressure zone forms over the sheet. While in heretofore known transfer devices attempts have been made to weaken the negative pressure arising from the relative speed between the sheet and the air located above it, in the transfer device according to the invention, the cause itself of a negative pressure zone arising over the sheet is removed, so that no negative pressure zone can form at all, or the negative pressure in the negative pressure zone is so low that lifting of the sheet or wavy contact of the sheet with the periphery of the cylinder is avoided.
The transfer device can be used in an advantageous way in a sheet-fed rotary printing machine for transferring sheets from a guide cylinder to a turning or reversing drum, where in-register sheet guidance is required in order to avoid rejects. In this exemplary embodiment, the cylinder is formed of a storage drum disposed in the region between the guide cylinder and the turning or reversing drum.
In a preferred embodiment, the air flow is inclined with respect to an imaginary tangent to the periphery of the cylinder at an angle xcex1, which is greater than or equal to 0xc2x0 and less than 90xc2x0, the tangent intersecting the point of impingement of a central flow line of the airflow. The airflow is therefore aligned in such a way that it strikes a sheet resting on the periphery of the cylinder at an acute angle or, in the case of an angle xcex1 equal to 0xc2x0, is directed parallel to the direction of displacement of the sheet in the region of the point of impingement in the direction of rotation of the cylinder.
In one exemplary embodiment of the transfer device, the invention provides for the airflow-generating device to generate compressed air or blast air. The velocity of the at least one airflow directed in the direction of rotation of the cylinder can preferably be matched to the travel speed and the material of the sheets, it being possible to realize higher velocities of the airflow with compressed air than with blast air.
A further development of the invention provides for the airflow-generating device to be associated with an air guide device which aligns the airflow. The airflow-generating device, which can be formed, for example, by a fan, a compressor or the like, therefore does not apply the compressed air or blast air directly to the sheet, but supplies it to the air guide device. The airflow can thereby be deflected precisely to a desired point on the peripheral region of the cylinder.
In a preferred embodiment, the air guide device has at least one blast box with one or more air outlet openings and/or at least one blast tube with at least one air outlet opening. The air outlet opening can be circular or slot-like. In an advantageous exemplary embodiment, the blast box/the blast tube extends over the entire length of the cylinder, so that the sheet can have applied thereto over the entire width thereof at least one airflow directed in the direction of rotation of the cylinder. Of course, it is also possible for a number of blast boxes and/or blast tubes to be arranged one after another, as viewed in the longitudinal direction of the cylinder, it being possible for the outlet openings thereof to be both circular and slot-like.
In order to achieve the objective, a method is also proposed. In the method, in order to transfer a sheet with the aid of a peripheral region of a first cylinder, and to feed the sheet in an exact position to a second cylinder arranged downline of the first cylinder, at least one airflow is directed onto that part of the peripheral region of the cylinder whereon the sheet is arranged. The method is distinguished by the fact that the air flow is directed in the direction of rotation of the cylinder. As a result, by comparison with heretofore known transfer devices, a lower differential speed between the sheet resting on the cylinder and the air located above it can be ensured, so that the negative pressure zone produced over the sheet by the only small relative speed between sheet and air is at least only so low that the formation of waves or creases in the sheet can be ruled out, and smooth contact between the sheet and the cylinder can be assured. In an advantageous embodiment, the velocity of the airflow and the speed of the sheet arranged on the peripheral region of the cylinder are equal, so that the formation of a negative pressure zone over the sheet can be prevented.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as a device and method for transferring a sheet, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein: